This invention relates to a novel resilient hydrophobic foamed polymer and a method of producing the same, particularly, to a polymer high in expansion degree, tensile strength and hardness and low in specific gravity and a method of producing the same. The polymer of this invention is suitable for use as a cushioning material.
In general a polyurethane foam is produced by a prepolymer method or one-shot method. In the prepolymer method, isocyanate terminated urethane prepolymer is prepared first by the reaction between polyetherpolyol and a molar excess of polyisocyanate, followed by reacting the prepolymer with water in the presence of, generally, an amine catalyst so as to obtain the product of polyurethane foam.
U.S. Pat. Nos. 3,812,618; 3,812,619 and 3,903,232 disclose a method of producing a polyurethane foam without using an amine catalyst. In these U.S. patents, a hydrophilic urethane prepolymer is formed first by the reaction between polyisocyanate and polyetherpolyol containing 60 to 100 mol % of oxyethylene, followed by reacting the prepolymer with a large amount of water so as to obtain the product of polyurethane foam.
Since the urethane prepolymers described in said U.S. patents present a solid form or an extremely viscous liquid form at lower temperatures than about 40.degree. C., considerable difficulties are encountered in uniformly mixing a large amount of water with such form of prepolymer. Therefore, manufacture of a foamed product from the above-mentioned prepolymer requires temperature control to maintain the prepolymer in the form of slightly viscous liquid, unavoidably resulting in high production cost. Furthermore, the resilient foamed polymers of said U.S. patent have the drawbacks that they have a relatively low degree of expansion on foaming, they have a dimensional shrinkage with time of about 15%, and are extremely hygroscopic. Consequently, they are considered unacceptable for use as cushioning materials, such as mattresses or automobile seats, or for use as heat-insulating materials.
Further, a method of producing a hydrophobic polyurethane foam, which comprises reacting polyisocyanate with hydrophobic polyetherpolyol such as polypropylene glycol so as to prepare a hydrophobic urethane prepolymer and reacting the prepolymer with water, is disclosed in, for example, U.S. Pat. No. 3,661,860. In this case, however, it is necessary to use an emulsifying agent for carrying out the reaction between the hydrophobic urethane prepolymer and water because a prepolymer of this type is incompatible with water.
Another method of producing a polyurethane foam is disclosed in U.S. Pat. No. 3,042,631. In this case, a prepolymer is formed first by the reaction among diisocyanate, polyetherpolyol containing 5 to 95 mol % of oxyethylene and polyether diamine containing 5 to 95 mol % of oxyethylene, followed by reacting the prepolymer with about 0.5 to 10% by weight of water so as to obtain the product of polyurethane foam. It should be noted that the oxyethylene content of the polyetherpolyol ranges as wide as between 5 and 95%. This suggests that the resultant prepolymer and polyurethane foam may be either hydrophobic or hydrophilic depending on the actual oxyethylene content of the polyetherpolyol. It should also be noted that a polyether diamine is used as a crosslinking agent or catalyst, resulting in that the product foam retains with an amine odor. An additional difficulty to be noted is that foaming cannot be carried out effectively and, thus, the cell size of the product foam becomes nonuniform because a small amount of water is reacted with the prepolymer.
A polyurethane foam for use as particularly a cushioning material should have a high hardness and a low specific gravity. It has been impossible, however, to produce a hydrophobic polyurethane foam meeting these properties.